1. Field of the Invention
This invention relates to radar systems and more particularly to a frequency modulated, phase coded radar system.
2. Description of the Prior Art
Prior radar systems generally fall into one of two classes. In the first class are pulse type systems in which a pulse of radio frequency energy is transmitted to a remote object and the return pulse is detected in the receiver. The time delay between the transmitted pulse and the return pulse is indicative of the range to the remote object. While measurement of this delay time can be made very accurately, the duty cycle of the system is too low and low duty cycle pulsed systems require high peak pulse power which results in high cost and low performance for electronic counter-counter measures. In order to substantially increase the duty cycle, a series of pulses is normally employed, but in order for these pulses to be distinguished, they must be coded in some manner. One form of such a pulse coded system is found in British patent 1,351,096 in which the transmitted signal is a periodically repeated sequence of binary pseudo random, square wave pulses. Another form of coding is described in British patent 1,208,582 which employs a magnetron to produce an output signal whose frequency is dependent upon the load imposed thereon. In order to encode the train of pulses emitted by the magnetron, the phase of the signal is reversed at predetermined points to produce a phase encoded signal.
The second class of radar system commonly found are frequency modulated systems which can have a nearly 100 percent duty cycle. One such frequency modulated system is shown in U.S. Pat. No. 4,107,679 in which a continuous wave oscillator is modulated by a linearly increasing frequency which changes from a first to a second frequency along a sloped ramp periodically. This signal is transmitted to the remote object and the reflected signal will be a similar signal whose frequency increases with time along the same sloped ramp but the return signal is delayed with respect to the transmitted signal by a time which is a function of the distance to the remote object. If the transmitted signal and the return signal are mixed, their difference is a fixed frequency signal whose magnitude is dependent upon the time delay and thus upon the distance to the remote object. Since it is difficult to measure the magnitude of this fixed frequency as an indication of distance, the slope of the ramp signal is continuously changed until a predetermined frequency is reached. Changing the slope of the frequency modulated ramp will also produce a change in the intermediate frequency from the mixer. This changing intermediate frequency is then provided to a narrow band filter from which an output will be obtained only when the output from the mixer is of the predetermined frequency. The time it then takes to sweep the frequency over the required frequency band is also indicative of the distance to the remote object and this can be detected more easily than detecting the actual frequency difference.
A difficulty encountered with the frequency modulated system described above is that linear ramps are quite difficult to generate and system accuracy will be severely affected by non-linearities in the transmitted ramp.
A difficulty for high duty cycle systems with separate transmitter and receiver antennas is that no matter how the transmitter is shielded from the receiver, some direct signal from transmitter to receiver still manages to get through and this becomes more and more of a problem at longer ranges when the reflected signal from the remote object becomes fainter and fainter. At some point, the direct signal from the transmitter to the antenna may produce a false range indication, which could be very dangerous.